Motown Direct Box DIY

[CJ]

12AU7a for stage 2 is the ticket. changed the resistors a bit, strapped the OPT for 600 to compensate for the reduced gain, took out the 1 MA pot and stuck in a 250 KA with a 270 K underneath for a a 50% to 100% vol control. (the 12AU7a likes a smaller grid resistor)

the use of different resistors for plate and cathode on the 2nd stage follower results in smooth clipping on top and bottom.

freq response is 20 -20 KC , output 1 to 2 volts p-p with a 100 mv p-p input signal.

messed up schemo and waveform at clipping level (1.2 v p-p input)

 

[capacitorless]

Following with interest. I have an ancient but viable Hammond organ amp chassis to try out this design. It would be an (edit: dual channel) octal build (6SL7/6SN7/6BX7) since the chassis is 7 holes of 8 pins apiece. Two hefty chokes and a hefty power transformer.

Given that I don't have a Triad HS-52 at the moment, can I replace that with a resistor load, and run a single-ended output from the MKT-1813 for testing purposes?

(edit: like this with less dust)

 

[CJ]

Yes, that's what I did, maybe a 100 k load resistor on the output side if the cap just for the heck of it,

 

[capacitorless]

Thanks, CJ! I'll pull the chassis out of storage and order some 6BX7 and a couple of caps.

 

[CJ]

Should sound good, those big tubes usually sound better than their 9 pin counterparts and the sockets are a lot easier to wire.

 

[capacitorless]

I just did a visual measurement of the one on Ebay, and it should be rack mountable to boot.

I just need to make sure the power transformer measures up with enough B+ (I plan to use sand rectification, so it should) and that the transformer/chokes don't generate too much EMI. I'll lay it out optimally though. The original output transformer and capacitor box will go, and I'll have the output transformers mounted on the end, as far enough away from the 2nd choke as possible. I might go with a single choke - they didn't have lots of microfarads available back then.

If y'all take a look at the gut shot of the one on Ebay, it's pretty nice to wire up. That means component swapping should be a breeze.

 

[CJ]

You might not need a choke, and yes, that 6000 turn choke I wound is generating some weird stuff so I am taking it out and adding an additional pu stage to make up for it,